1. Field of the Invention
The present invention relates to a field sequential color imaging apparatus which uses an X-Y addressed solid state imaging device.
2. Description of the Prior Art
In order to obtain a color picture by an endoscope which uses a solid state imaging device, it is known to switch over the light from a light source to different primary colors for each of several fields, and to reproduce a color image by primary color signals transmitted in succession in a field sequential manner. This field sequential color imaging system has an advantage in that it produces an image of a high resolution with fewer picture elements, when compared with a system which employs an imaging device with a color filter array provided on the device.
U.S. Pat. No. 4,253,447 discloses an endoscope which employs a field sequential color system of the above-described type.
In this endoscope, an object is illuminated by lights of different red, green and blue wavelengths, and the signals obtained by a charge-coupled device under the individual lights are transmitted at times during which the light illumination is suspended and are stored in frame memories.
However, if a solid state imaging device which is of an X-Y addressed type such as a MOS type or a charge modulation device which is described from page 353 to 356 of IEDM Digest of Technical Papers issued in 1987 is used, special attention must be paid in illuminating the light from the light source. More specifically, in either of the above-described devices, the exposure of individual picture elements starts at the time at which they are scanned, so that the exposure period is different for each picture element. Therefore, if the imaging screen is continuously scanned while the primary colors are switched over for each field, the primary color signals which represent individual picture elements are mixed at a different ratio for each picture element to form a signal representing each picture element. In order to obtain a single primary color signal for each of the picture elements in each field, the scanning must be suspended while each primary color is illuminated, the image being scanned so that its signals are read during the suspension of light illumination. In other words, it is necessary to provide a period during which light illumination is suspended for each field, as in the endoscope disclosed in U.S. Pat. No. 4,253,447.
The above-described system, however, suffers from problems involving an inefficient use of light and a lowered sensitivity. More specifically, provision of the period during which the light illumination is suspended reduces the intensity of light which is illuminated, thereby reducing the sensitivity. In particular, when the number of picture elements is increased so as to reproduce an image of a higher resolution, the period during which the signals are read must be made correspondingly longer, further reducing the sensitivity. Deterioration of the sensitivity may be prevented by increasing the period of one field. However, this increases the degree of color shift which occurs when the object moves, and makes reproduction of a good quality image impossible. The period during which the light illumination is suspended may be shortened by increasing the signal read-out frequency of the device. However, there is a limitation to this in terms of the operation of the device.